US-based medical diagnostics company Quest Diagnostics (NYSE: DGX) has closed the acquisition of US-based cardiovascular disease management firm Cleveland HeartLab from equity investors, including Cleveland Clinic, the company said.This deal was announced in October. Quest said it plans to establish a national center of excellence in diagnostic information services at Cleveland HeartLab's specialised laboratory in Cleveland, Ohio, focused on services that aid in assessing risk and treatment protocols for heart disease, the leading cause of death in the United States.